Energy storage device with large charge separation

ABSTRACT

High density energy storage in semiconductor devices is provided. There are two main aspects of the present approach. The first aspect is to provide high density energy storage in semiconductor devices based on formation of a plasma in the semiconductor. The second aspect is to provide high density energy storage based on charge separation in a p-n junction.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. application Ser. No.13/135,798, filed Jul. 13, 2011, and hereby incorporated by reference inits entirety. Application Ser. No. 13/135,798 claims the benefit of U.S.provisional patent application 61/399,574, filed on Jul. 13, 2010,entitled “Energy Storage Device with Large Charge Separation inStructures Containing PN Junctions”, and hereby incorporated byreference in its entirety. Application Ser. No. 13/135,798 also claimsthe benefit of U.S. provisional patent application 61/399,757, filed onJul. 15, 2010, entitled “Multiple Exciton Generation and Storage in anEnergy Storage Device with a Narrow Bandgap Semiconductor”, and herebyincorporated by reference in its entirety. Application Ser. No.13/135,798 also claims the benefit of U.S. provisional patentapplication 61/520,960, filed on Jun. 17, 2011, entitled “All ElectronBattery with an Electron Plasma for Charge Storage and Permittivity”,and hereby incorporated by reference in its entirety.

GOVERNMENT SPONSORSHIP

This invention was made with Government support under contract numberW911NF-07-2-0027 awarded by the US Army Research Laboratory, and undercontract DE-AR0000069 awarded by ARPA-E under the Department of Energy.The Government has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to energy storage.

BACKGROUND

Energy storage is a crucial component of a large number and variety ofelectronic devices, particularly for mobile devices and electric orhybrid vehicles. Energy storage devices have been based on a widevariety of physical effects. For example, electric fields can beemployed to store energy in capacitors, and chemical reactions(involving ion motion) can be employed to store energy in batteries.However, energy storage in a capacitor can be limited by the devicegeometry (e.g., 2-D capacitor plates having limited area), and batteriescan have a slow response time due to the ion motion inherent inelectrochemical reactions.

Battery powered devices such as hybrid or electric vehicles are oftenlimited in performance by the low energy stored per weight in batteries.Batteries have low storage density due to the low voltage produced byelectrochemical reactions and the large size and weight of the ionsstored in the batteries. Slow ion transport in batteries also causesslow charge and discharge performance. Furthermore, the reliance ofexisting batteries on ionic transport causes high degradation rates ofthe batteries.

Accordingly, it would be an advance in the art to provide energy storagehaving higher energy density than a capacitor, faster charge/dischargethan a battery and/or much longer lifetime than a battery.

SUMMARY

In the present work, high density energy storage is provided insemiconductor devices. There are two main aspects of the presentapproach. The first aspect is to provide high density energy storage insemiconductor devices based on formation of a plasma in thesemiconductor. The second aspect is to provide high density energystorage based on charge separation in a p-n junction.

In one embodiment, a plasma based device includes at least onesemiconductor layer with a low bandgap, at least one barrier layer withhigh breakdown field strength (and high resistivity and bandgap), and ananode and cathode. A strong electrical field, nearly as high as thebreakdown voltage in the barrier layer, is applied between the anode andcathode, polarizing the material in between. The low bandgapsemiconductor becomes polarized as the electric field separateselectrons and holes in the material by direct excitation by the E-fieldacross the bandgap.

If the polarization is large enough and very high charge density isobtained, an electron and/or hole plasma may form. In the plasma, thescreening length increases with the charge density, thereby reducing therepulsion between the charges, allowing the charge density to be furtherincreased. At the same time, the permittivity of the plasma increases,and may reach very high values (>10̂4) if the charge density is high andthe length scale over which the charge density is nearly uniform islarge (>10 nm). Uniform planar layers may be fabricated over distancesof >>100 nm, so uniformity of charge distribution over large areas maybe achievable with thin film processing. Further, the density of statesof the semiconductor may be engineered to allow a group velocity that isadvantageous to plasma formation. By realizing a device with highpermittivity and high charge concentration, the device will achieve veryhigh energy density. For the plasma approach, semiconducting materialsare generally narrow-bandgap semiconductors (bandgap preferably lessthan 1.0 eV, more preferably less than 0.3 eV), for example, PbS, PbSe,InSb, InAs, PbTe, GaSb, and Hg_(x)Cd_(1-x)Te.

Together with polarization in the semiconductor, there may be chargeinjection from the electrodes into the low bandgap regions. This mayfurther increase the charge density, thereby increasing energy storage.Low cost may be obtained since most of the volume of the device (thethick semiconductor layer(s)) does not have to be deposited with highquality. To obtain high E_(bd) values, the barrier material may berequired to be of high quality.

In a second aspect, charge separation in a p-n junction is exploited toprovide energy storage. This approach creates a large polarization of amaterial in response to applying a voltage across electrodes on eitherside of the material. Instead of a dielectric material, the presentapproach uses one or many layers of p-n junctions, surrounded on atleast one side by a wide-bandgap insulating material. For the p-njunction approach, p and n materials are generally wide-bandgapsemiconductors (bandgap preferably greater than 0.5 eV, more preferablygreater than 1.5 eV) with a charge carrier density of at least 10¹⁰/cm³,for example, Si, Ge, GaAs, InP, InAs, TiO₂, ZnO, and ZnS. The insulatingmaterial generally has a high breakdown voltage, and high resistivity,and a wide bandgap. Materials such as Al₂O₃, SiO₂, or HfO₂ are suitablefor the insulating material.

The energy density of a capacitor with a relative dielectric constantepsilon and breakdown field strength of E_(BD) is given by½*epsilon*epsilon_0*E_(BD) ², if epsilon is a constant independent ofthe electric field, where epsilon_0 is the permittivity of free space.According to the present approach, to achieve a high epsilon, andtherefore a high energy density, the polarizability of the material isgenerally high. That is, an applied electric field will cause a largenumber of charges to separate over large distances.

If epsilon is not a constant with electric field (i.e. ∈=∈(E)), theenergy density of a capacitor is given by

∫₀^(E_(bd))ɛ(E)EE.

Therefore, a dielectric including a material with a permittivity thatincreases with field will store additional energy compared to a materialwith a lower and constant permittivity. Aspects of the present approachembody methods to create a material that behaves as a dielectric with apermittivity that increases with applied field, in contrast to naturallyoccurring insulators, which have a permittivity which is constant ordecreasing as field strength increases.

According to principles of the invention, when an electric field isapplied across the electrodes, the carriers flow in response, separatingover relatively large distances that are comparable to the thickness ofthe p+n layers. Therefore, a large polarizability and high energydensity is provided. A high charge carrier mobility in the p and nlayers is preferred to achieve a large charge displacement in responseto an electric field.

Accordingly, the semiconductor layers should have high mobility,preferably above 10⁻⁴ cm²/Vs. Several methods to measure carriermobility are known, including Hall effect measurements, and field-effectmobility measurements in the saturation and/or linear regions.Explicitly, carrier mobility may be determined by doing a Hall effectmeasurement and finding the mobility μ=−σ_(n)*R_(Hn), where σ_(n) is theelectron conductivity, which is tabulated or measured, and R_(Hn) is theelectron Hall coefficient. Along with high mobility, the charge carriersshould have a low effective mass so that they are more effectivelyaccelerated by an electric field, giving a high polarizability. Thecharge carrier effective mass is preferably below 0.2m_(e), where m_(e)is the mass of a free electron.

The availability of charge carriers is another important feature of thesemiconductor materials to achieve high polarizability, therefore thecarrier concentrations of the p and n materials should be high andapproximately equal. An alternative means to achieve a high carrierconcentration is to have a material Fermi level near the valence orconduction band level, or a doping level sufficient to have a carrierconcentration preferably above 10¹⁹/cm³, and even more preferably above10²¹/cm³. The carrier concentration can be measured, for example byoptical means or by a Hall effect measurement.

Variations of the p-n junction approach can include devices whereseveral layers of (ipn) are included in a row (e.g. electrode|ipn ipnipn i|electrode). In such devices, additional effects may occur acrossthe insulator layers in the middle. Since they separate n and p layerswith a layer of insulator that may be as thin as 1-10 nm, twointeraction effects across that layer may occur. First, the interactionis attractive, since it occurs between charge carriers of differentsign. This attractive interaction makes the polarization easier, andtherefore increases the effective epsilon of the material. The thinnerthe insulator layer is, the greater the attractive interaction betweenelectrons and holes on opposite sides of the insulator, and thereforethe charge density on each side of the dielectric may be greater. Theenergy density is therefore increased since more charges may be storedat the same voltage in the device. An embodiment of the invention mayinclude insulating layers of varying thicknesses throughout the device,such as a continually increasing insulator thickness (or a continuallyincreasing insulator bandgap) throughout the device so that it becomesincreasingly difficult for charge carriers to tunnel across theinsulating regions, so that some charge tunneling is allowed near theelectrodes but charges are eventually blocked from tunneling through theentire device, which would result in a device that is electricallyshorted.

The second interaction effect across the insulator is when formation ofexcitons across the barrier occurs. When these excitons are kept forrelatively long periods of time (larger than the recombination timescale), at low temperatures or high densities, a condensation may occur(Bose-Einstein condensation (BEC)). To improve the chance of BECformation, the Bohr exciton radius of the semiconductor materials shouldbe large, preferably over 5 nm, more preferably over 10 nm, and stillmore preferably over 15 nm. A further effect that may be employed toincrease the chance of BEC formation is to apply a magnetic fieldperpendicular to the interface of the semiconductor materials. When themagnetic field has a strength such that the Landau levels arehalf-filled (an occupation of ½ electrons in one material and holes inthe other material), BEC formation is optimal.

Another embodiment has the insulating region replaced with a tunneljunction including heavily doped n+ and p+ regions as in a solar cellwith tunnel junctions. The tunnel junctions provide a method to increasethe voltage of a single device by effectively putting several devices inseries. Electrons from one region may tunnel through the tunnel junctionto combine with holes from the adjacent region. Therefore, a continuityof current is achieved while the voltage is stacked.

Diode p-n junctions in reverse bias have a junction capacitance due tocarrier depletion at the interface. That capacitance, however, isdecreasing with voltage, whereas the capacitance in structures of thepresent approach feature a capacitance that increases with voltage.

The present approach provides numerous advantages, and is widelyapplicable. Some of the advantages of embodiments of the inventioninclude: low cost materials and fabrication compared to other energystorage approaches, higher energy density compared to capacitors, highersafety, durability, charge/discharge rate, lifetime, roundtripefficiency, and/or power density compared to batteries.

Applications include energy storage for portable electronics,transportation, automotive, grid/utility scaled applications, and thelike. Further, this invention may be useful for providing decouplingdevices for integrated circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a-c show examples of semiconductor plasma devices for storingenergy.

FIG. 2 shows an alternative geometry for a semiconductor plasma device.

FIG. 3 shows an approach for charging a semiconductor plasma device.

FIGS. 4a-c show calculated screening parameters and dielectric constantsfor a numerical example.

FIG. 5 shows the configuration of an experimental semiconductor devicedemonstrating enhanced energy storage.

FIG. 6a shows experimental results from a control device.

FIG. 6b shows experimental results from a semiconductor energy storagedevice sample.

FIGS. 7a-b show simulation results assuming the semiconductor of theexample of FIG. 6b acts as an insulator (FIG. 7a ) or as a conductor(FIG. 7b ).

FIGS. 8a-c show operation of a semiconductor p-n junction device forstoring energy.

FIGS. 9a-b show further examples of semiconductor p-n junction devicesfor storing energy.

FIG. 10 shows another example of a semiconductor p-n junction device forstoring energy.

FIG. 11 shows an example of a semiconductor p-n junction device forstoring energy where discharged energy is emitted optically.

DETAILED DESCRIPTION A) Plasma Formation for Energy Storage

FIGS. 1a-c show examples of semiconductor plasma devices for storingenergy. In the example of FIG. 1a , a first volume of active material102 is disposed between conductive electrodes 106 and 108. Uponapplication of a voltage across the electrodes, one or both of anelectron plasma or hole plasma is formed in the first volume of activematerial 102, according to above-described principles. The electronplasma and/or hole plasma independently have a density of chargecarriers of greater than about 0.5 electron (or hole) per nm³ when thematerial is subject to a field of 0.7 V/nm. Preferably, these carrierconcentrations are higher (e.g., greater than about 1.0 electron (orhole) per nm³).

Here and throughout this description, it is convenient to describeapplication of an electric field to a material in terms of the externalvariables of applied voltage and material thickness. Thus, an appliedfield of 0.7 V/nm can be obtained by providing 0.7 V across 1 nm ofmaterial, 0.35 V across 0.5 nm of material, etc. Typically, the electricfield within a material that is subject to such an applied field willvary with position within the material, and this electric field may ormay not equal the applied field at any point within the material. Thevoltage applied across the electrodes to form the plasma in the firstvolume of active material 102 produces an energy density of greater thanabout 1 Wh/L for the device.

Energy density may be measured by constant current charge-dischargemeasurements of the voltage as a function of charge state (galvanostaticmeasurement). In this measurement, a constant current of one polarity isapplied while the voltage across the device increases to the maximumrated voltage, then the polarity of the current is reversed until thedevice voltage returns to zero. Energy density during charge is derivedfrom the area under the curve of a plot of the charge versus voltage(charge q(t)=I(t)*dt) during the charging cycle, and energy densityduring discharge is calculated similarly during the discharge portion ofthe cycle. Alternatively, the energy density can be measured by applyinga constant voltage ramp up to the maximum voltage and measuring thecurrent, then applying a voltage ramp of the opposite polarity untilvoltage returns to zero and measuring the current. Again,energy=∫q(V)dV=∫V(q) dq.

Preferably, as shown on FIG. 1a , a volume of barrier material 104 isdisposed between active volume 102 and an electrode. The composition anddimensions of barrier volume 104 are preferably selected such that thereis a range of applied fields to the device such that a) an electronand/or hole plasma forms in active volume 102, and b) the electric fieldin barrier volume 104 is less than the breakdown field strength of thematerial of barrier volume 104. Preferably, the breakdown field strengthof barrier volume 104 is greater than about 0.5 V/nm. Suitable materialsfor barrier layer 104 include but are not limited to: SiO₂, Al₂O₃, HfO₂,Pr₂O₃, nitrides, fluorides, diamond, Zr:HfO₂, SiON, Pb(ZrTi)O₃, andcombinations thereof.

In the example of FIG. 1a , charge may be injected from electrode 108into active volume 102. Charge injection is preferable if screening inthe active volume is higher than screening in the electrode and/or ifthe barrier volumes can withstand a larger electric field. Chargeinjection into the active volume may be measured by investigation ofoptical properties of the active volume, such as transmission orabsorption. In other cases, it may be desirable to prevent such chargeinjection, as in the example of FIG. 1b . Here active volume 102 issandwiched between barrier volumes 104 a and 104 b. In this example, itis preferred for plasma to form in active volume 102 at an appliedvoltage to electrodes 106 and 108 that is sufficiently low thatbreakdown does not occur in barrier volumes 104 a and 104 b. Morespecifically, electric fields produced in barrier volumes 104 a and 104b in response to the applied voltage are preferably below the breakdownfield strengths of the materials of barrier volumes 104 a and 104 b.Preferably, the breakdown field strengths of barrier volumes 104 a and104 b is greater than about 0.5 V/nm. Suitable materials for barrierlayers 104 a and 104 b include but are not limited to: SiO₂, Al₂O₃,HfO₂, Pr₂O₃, nitrides, fluorides, and combinations thereof.

Preferably, active volume 102 is a semiconducting material, whichpreferably has a band-gap energy of less than about 1 eV (morepreferably, a band-gap energy of less than about 0.3 eV). Preferably,the effective mass of the charge carriers in the electron-plasma orhole-plasma of active volume 102 is less than about 0.2 m_(e), wherem_(e) is the electron mass. The effective mass may be measured bycyclotron resonance or band-structure measurements, where the electroneffective mass is determined from the curvature of the conduction band.Preferably, active volume 102 has a dielectric constant of less thanabout 20. Suitable semiconducting materials for active volume 102include, but are not limited to: PbS, PbSe, InSb, InAs, PbTe, GaSb andHg_(x)Cd_(1-x)Te. Of these materials, PbSe, InSb, Hg_(x)Cd_(1-x)Te andInAs are preferred, and InSb is particularly preferred at the presenttime.

Preferably, all linear dimensions of active volume 102 are greater thanabout the bohr radius of an exciton in the material of active volume102. Other preferred embodiments have all linear dimensions of activevolume 102 greater than about 100 nm. In some cases, active volume 102is planar in shape and has a thickness of less than about 3 μm;

Multi-layer embodiments are also possible. For example, the combinationof volumes 102 and 104 on FIG. 1a can be regarded as a unit cell, andthis unit cell can be repeated one or more times between the electrodesto form a multi-layer structure. In such multi-layer structures, each ofthe active volumes may be the same material and/or different materials.Similarly, the compositions of the barrier volumes may be the sameand/or different. The thicknesses of the semiconducting materials andbarrier materials may be the same and/or different.

FIG. 1c shows an embodiment having two active volumes 102 a and 102 bsandwiching a barrier volume 104. Preferably, active volumes 102 a and102 b of this example are as described above in connection with activevolume 102 on FIG. 1a , and barrier volume 104 here is preferably asdescribed above in connection with barrier volume 104 on FIG. 1a . Insome preferred embodiments, the thickness of barrier volume 104 on FIG.1c is greater than about 5 nm, resulting in a separation between activevolumes 102 a and 102 b of greater than about 5 nm. In some embodiments,configurations as in FIG. 1c can have carrier injection from electrode106 to active volume 102 a and/or from electrode 108 to active volume102 b in response to an applied voltage at the electrodes. The Fermilevels of active volumes 102 a and 102 b can be the same or they can bedifferent.

Any and all known semiconductor fabrication technology can be employedto fabricate plasma-based energy storage devices as described herein.For example, layers may be fabricated using thin film depositiontechniques such as MBE, PLD, ALD, sputtering, CVD, MOCVD, chemical bathdeposition, or a layer transfer process. Such fabrication is within theskill of an art worker, and is therefore not described in detail here.

FIG. 2 shows an alternative geometry for a semiconductor plasma device.In this example, electrode 208 is cylindrically shaped and encloseselectrode 206. Between the electrodes, an active volume 202 and abarrier volume 204 are disposed. Preferably the active and barriervolumes of this example are as described above in connection with FIGS.1a-c . An advantage of this approach is that electric field linesconcentrate, as schematically shown by 210, which may assist with plasmaformation.

FIG. 3 shows an approach for charging a semiconductor plasma device. Inthis example, a device as in FIG. 1a is charged by the combination of anapplied voltage from a voltage source 302 and electromagnetic radiation304. Preferably, the electromagnetic radiation has photon energy greaterthan the band gap energy of active volume 102. This approach may assistwith carrier generation, and therefore plasma formation.

Screening of electrons in a plasma allows a larger charge density byreducing the repulsive force due to carrier-carrier interaction. Onemodel for screening is given by

${V_{s}(r)} = {\frac{e^{2}}{ɛ_{0}r}e^{{- \kappa}\; r}}$

where V_(s) is the screened electrostatic potential energy, e is theelectron charge, ∈₀ is the permittivity of free space, κ is thescreening parameter, and r is distance. The screening parameter in 3-Dis given by

$\kappa_{3D} = \sqrt{\frac{4\pi \; e^{2}}{ɛ_{0}}\frac{n}{\mu}}$

where n is the carrier concentration and μ, is the Fermi level (alsoknown as the chemical potential). At sufficiently low temperatures,κ_(3D) is approximately given by

$\kappa_{3{Dapx}} = \sqrt{\frac{6\pi \; e^{2}n}{ɛ_{0}E_{f}}}$

where E_(f) is the Fermi energy referenced to the vacuum potential. Atroom temperature, this approximation typically holds to several decimalplaces. In 2-D, the screening parameter is given by

$\kappa_{2\; D} = {\frac{2{me}^{2}}{ɛ_{0}\hslash^{2}}\left( {1 - {\exp \left( {{- {\pi\hslash}^{2}}{n/{mkT}}} \right)}} \right)}$

where m is the effective mass of the carriers.

FIG. 4a shows some plots of the screening parameter vs. carrierconcentration. The solid line shows calculated results for 2-D screeningat room temperature. The dashed and dotted lines show κ_(3Dapx) for twodifferent values of Ef (E_(f)=5 eV for the dashed line and E_(f)=2 eVfor the dotted line).

For a plasma, the dielectric constant is given by:

$ɛ_{2D} = {1 + \frac{\kappa}{q}}$$ɛ_{3D} = {1 + \frac{\kappa^{2}}{q^{2}}}$

where ∈_(2D) is the dielectric constant for a 2-D plasma, ∈_(3D) is thedielectric constant for a 3-D plasma, κ is the screening parameter asdescribed above, and q is the change in momentum experienced by anelectron when a perturbation (in this case, the electric field) isapplied (q=k_(f)−k_(i)). By engineering the density of states of thesemiconductor material, one may select for a small q. One may engineer amaterial with a density of states high enough that states becomedegenerate and the momentum transfer on scattering is small, thereforethe effective permittivity is high. The group velocity v_(g) is foundfrom the dispersion relation of the solid: v_(g)=∂Ω/∂k. Therefore, thegroup velocity goes to zero when states become degenerate and momentumtransfer becomes small.

An interesting feature of the equation for the screening length κ_(3D)above is that the screening length increases with n, the carrierdensity. Since the permittivity increases with the screening length, andthe carrier density increases with the applied voltage, this results inan effective permittivity that increases as a function of the electricfield. An increasing permittivity with field is advantageous for energystorage as discussed above and is not often found in naturally occurringmaterials and therefore is an attractive and distinguishing feature ofthe present approach.

The plasma theory developed above considers an electron gas subject to aperturbation. The solution is a ground state solution, meaning it is astable state of the perturbed system, thus a long lifetime can beexpected in the state, which is suitable for energy storage.

Since quantum wells in 2-D can be fabricated with homogeneity >100 nm,large areas may be fabricated with a high dielectric constant, andtherefore energy storage. FIGS. 4b and 4c show calculated 3D and 2Ddielectric constants, respectively, for various values of κ and q. Withκ on the order of 1 nm⁻¹ and q on the order of 0.01 nm⁻¹, ∈ values over100,000 should be possible for both 2D and 3D cases.

FIG. 5 shows the configuration of an experimental semiconductor devicedemonstrating enhanced energy storage.

In this example, the experimental device included an evaporated Al topelectrode 502 (of diameter 1 mm), a 130 nm thick PbS layer 504(fabricated by PLD), a 30 nm Al₂O₃ layer 506 (fabricated by ALD), ap-doped Si substrate 508, and a Cu tape bottom electrode 510. A controldevice (not shown) has the same configuration except that the PbS layeris removed. Current-voltage measurements were taken in response to 0.01s voltage transients having several different amplitudes. The shape ofthese voltage transients is a single-peak triangle wave.

FIG. 6a shows experimental results from the control device. Theseresults are consistent with a simple RC model, as shown by dotted linesgenerated by an RC model with R and C values consistent with expectedvalues of Al₂O₃ (a permittivity of 8.5). FIG. 6b shows experimentalresults from the sample of FIG. 5. Substantially more current flow(i.e., more charge/energy storage) is seen here than in the controldevice results of FIG. 6a . FIGS. 7a-b show simulation results assumingthe PbS layer of the example of FIG. 5 acts as an insulator (FIG. 7a )or as a conductor (FIG. 7b ). In both cases, the simulation resultsdiffer from the experimental results by orders of magnitude, which makethis a surprising and unexpected result. Apparently, this sample is ableto store much more charge/energy than one would expect from simpleconventional capacitance models. It is believed that the above-describedplasma effect may account for this result. The sample of FIG. 5demonstrated a discharge energy density of approximately 7 Wh/L and acharge-discharge efficiency above 80%.

B) p-n Junctions for Energy Storage

In another aspect of the present approach for providing energy storage,semiconductor p-n junctions can be used for energy storage. FIGS. 8a-cshow operation of a semiconductor p-n junction device for storingenergy. In this example, an active volume is disposed between conductiveelectrodes 106 and 108. The active volume has a first region 802 n and asecond region 802 p, where the two regions have differing Fermi levels.Preferably, one of the two regions is doped n-type (802 n) and the otheris doped p-type (802 p). Application of a voltage greater than 5 Vacross the electrodes produces an energy density of greater than 1 Wh/Lin the device. Regions 802 n and 802 p can be of the same material(homojunction) or of different materials (heterojunction). The activevolume is separated from the electrode by barrier volumes 804 a and 804b.

FIG. 8a shows charging of the device. A voltage source 810 chargeselectrodes 106 and 108 as shown. These charges induce a chargeseparation in the active volume (e.g., electrons in n-type region 802 ngo left on FIG. 8a , and holes in p-type region 802 p go right on FIG.8a ). The resulting configuration of the active volume can have largecharge separation over a large distance, which is conducive to highenergy storage.

FIG. 8b shows an open-circuit, or disconnected state (i.e., an energystoring state). Because the electrons and holes are at opposite sides ofthe active volume, this configuration is capable of storing energy forsignificant lengths of time. The main loss mechanism is expected to becurrent leakage through barrier volumes 804 a and 804 b, which can bemade low by suitable barrier volume design (e.g., choice of material andbarrier volume thickness) according to known principles for selectinggood insulators. FIG. 8c shows discharging of the device through a load812. Charge flow and current flow is reversed relative to the chargingconfiguration of FIG. 8a , so the stored energy is delivered to load812.

The active volume can include any semiconducting material or materials.Suitable materials include, but are not limited to: Si, Ge, GaAs, InP,InAs, TiO₂, ZnO, ZnS, and combinations thereof. Preferably, high dopinglevels are employed (charge carrier density preferably greater thanabout 10¹⁹ carriers per cm³, more preferably greater than about 10²¹carrier per cm²). The effective mass of the charge carriers ispreferably less than 0.2 m_(e), where m_(e) is the electron mass.Preferably, the active volume semiconducting materials have relativelylarge band gap energy (band gap energy preferably greater than 0.5 eV,more preferably greater than 1.5 eV). In cases where several differentmaterials are employed in the active volume (e.g., a heterostructure),it is preferred for each of the materials to independently have carrierdensity, effective mass and band gap as described immediately above. Insome preferred embodiments, the active volume is planar in shape withall linear dimensions less than about 200 nm.

In the example of FIG. 8a , it is preferred that application of avoltage sufficient to produce an energy density of greater than about 1Wh/L in the device produces electric fields in breakdown volumes 804 aand 804 b that are less than the respective breakdown field strengths ofthe barrier volume materials. Any suitable insulating material can beused for the barrier volume. The barrier volume material preferably hasa band gap energy of about 6 eV or more, and preferably has a breakdownfield strength of greater than about 0.5 V/nm. Suitable materialsinclude, but are not limited to: SiO₂, Al₂O₃, HfO₂, Li₂O, SiON, Zr:HfO₂and fluorides.

Any and all known semiconductor fabrication technology can be employedto fabricate p-n junction based energy storage devices as describedherein. For example, layers may be fabricated using thin film depositiontechniques such as MBE, PLD, ALD, sputtering, CVD, MOCVD, chemical bathdeposition, or a layer transfer process. Doping may be done by ionimplantation or from the precursors, targets, or source materials in anyof the above approaches. Such fabrication is within the skill of an artworker, and is therefore not described in detail here.

In some embodiments, the device further includes a modifying volume thatis disposed proximate to the active volume and which alters a Fermilevel of the active volume. When two bodies of different Fermi levelsare adjacent, or close enough to establish an equilibrium, electronand/or ion transfer between the two bodies occur to reachelectrochemical equilibrium. For example, electrons transfer from amaterial of low work function to one of high work function to minimizeenergy, if they are able. The new state thereby established featuresmaterials of modified electrochemical potential, or Fermi level. Thistechnique may be applied to modify the electronic structure of layersdescribed in embodiments of the present approach to enhance the energystorage capacity, efficiency, power density, or other features ofdevices made using the present approach.

FIGS. 9a-b show further examples of semiconductor p-n junction devicesfor storing energy. Both of these examples can be regarded as instancesof multi-layer device structures. In the example of FIG. 9a , a firstactive volume (including regions 802 n 1 and 802 p 1) and a secondactive volume (including regions 802 n 2 and 802 p 2) are separated by abarrier volume 804 b. This sub-assembly is sandwiched between electrodes106 and 108, and separated from the electrodes by barrier volumes 804 aand 804 c. The example of FIG. 9b is similar to that of FIG. 9a , exceptthat the central barrier volume 804 b is not present. Instead, a tunneljunction is disposed between region 802 p 1 of the first active volumeand region 802 n 2 of the second active volume. This tunnel junctionincludes a thin heavily doped p-type layer 902 and a thin heavily dopedn-type layer 904. These structures can be repeated any number of timesbetween the electrodes. For example, let E represent an electrode, Arepresent an active volume, B represent a barrier volume, and Trepresent a tunnel junction. Then possible structures include but arenot limited to: EBABABE, EBATABE, EBABATABE, EBABABABE, etc. Anintermediate barrier volume prevents the flow of current, so a chargeseparation can occur across such a barrier volume, as in the example ofFIG. 9a . In contrast, a tunnel junction permits current to flow, so itprovides a way to stack up voltage in series without blocking the flowof current. In such multi-layer structures, each of the active volumesmay be the same material and/or different materials. Similarly, thecompositions of the barrier volumes and tunnel junctions may be the sameand/or different.

In the example of FIG. 9a , it is preferred that application of avoltage sufficient to produce an energy density of greater than about 1Wh/L in the device produces electric fields in barrier volumes 804 a,804 b, and 804 c that are less than the respective breakdown fieldstrengths of the barrier volume materials. Preferably, the activevolumes and barrier volumes in multi-layer structures are as describedabove in connection with FIGS. 8a-c . Tunnel junctions suitable formaking ohmic contact between oppositely doped semiconducting layers arewell known in the art (e.g., multi-junction solar cells), and suchtechniques are applicable here. As described above, excitons may formand possibly condense at intermediate barrier volumes (e.g., 804 b onFIG. 9a ). Preferably, the separation between the first and secondactive volumes in the example of FIG. 9a is greater than 5 nm and morepreferably greater than about 10 nm. The barrier layer must be thickenough to substantially block current flow, for instance, due to quantumtunneling, which may occur through thin layers.

The preceding description has been by way of example as opposed tolimitation. Many variations of the given examples can also be use topractice embodiments of the invention. For example, the p-n junctionapproach can be practiced with barrier volumes on both sides of theactive volume (e.g., as shown on FIG. 8a ), or with a barrier volume onone side of the active volume. FIG. 10 shows an example of thisconfiguration, which is like the example of FIG. 8a , except thatbarrier volume 804 b of FIG. 8a is omitted.

Another variation relates to how stored energy is released from a p-njunction energy storage device. In the example of FIG. 8c , storedenergy is released by flow of electrical current through an electricalload 812. Alternatively and/or in combination, stored energy in such adevice can be released by emission of electromagnetic radiation, as inthe example of FIG. 11. In this example, discharge of the device entailselectron-hole recombination in the junction between regions 802 n and802 p, which can lead to emission of electromagnetic radiation 1102.Optionally, a photovoltaic device 1104 can be disposed to receiveradiation 1102 and provide electrical power as an output. Use ofphotovoltaic device 1104 is preferred in cases where the stored energyis to be provided as electrical energy as opposed to radiative energy.Radiation 1102 can be converted to electrical energy by photovoltaicdevice with high efficiency because radiation 1102 is band gap radiationhaving a relative small range of wavelengths (e.g., compared tosunlight). Thus, photovoltaic device 1104 is preferably tuned tomaximize efficiency for the wavelength band of radiation 1102.

Further variations that may relate to embodiments of the presentinvention are described in U.S. provisional application 61/506,592,entitled “Solid State Energy Storage Devices”, filed on Jul. 11, 2011,and hereby incorporated by reference in its entirety.

1. An energy storage device comprising: first and second conductiveelectrodes spaced apart; a first volume of active material disposedbetween the electrodes; a first volume of barrier material disposedbetween the first electrode and the first volume of active material;wherein the first volume of active material is a semiconducting materialcomprising a first region having a first Fermi level and a second regionhaving a second Fermi level different from the first Fermi level; andwherein application of a voltage of greater than 5 V across theelectrodes produces an energy density of greater than about 1 Wh/L inthe device.
 2. The device of claim 1, further comprising: a secondvolume of active material disposed between the second electrode and thefirst volume of active material.
 3. The device of claim 2 furthercomprising: a second volume of active material and a third volume ofbarrier material disposed between the first and second volumes of activematerial; wherein the second volume of active material comprises a thirdregion having a third Fermi level and a fourth region having a fourthFermi level different from the third Fermi level; wherein the barriermaterial in the third volume of barrier material has a third breakdownfield strength, and wherein application of the voltage across theelectrodes to produce an energy density of greater than about 1 Wh/L inthe device produces an electric field in the third volume of barriermaterial less than the third breakdown field strength.
 4. The device ofclaim 3, wherein the distance between the first and second volumes ofactive material is greater than about 5 nm.
 5. The device of claim 3,wherein the distance between the first and second volumes of activematerial is greater than about 10 nm.
 6. The device of claim 1, furthercomprising one or more additional volumes of barrier material and one ormore additional volumes of active material, wherein the volumes ofbarrier material and volumes of active material are alternatinglydisposed between the electrodes, and wherein each additional volume ofactive material has two regions having differing Fermi level.
 7. Thedevice of claim 1, further comprising one or more additional volumes ofactive material and one or more tunnel junctions, wherein eachadditional volume of active material has two regions having differingFermi level, and wherein the volumes of active material and the tunneljunctions are alternatingly disposed between the electrodes.
 8. Thedevice of claim 1, wherein the device is configured to release storedenergy by emission of electromagnetic radiation.
 9. The device of claim1, wherein the device is configured to release stored energy by flow ofelectrical current through an electrical load.